Intergranular susceptibility in failures of high pressure tubes

MASSONE, J.; OTEGUI, J. L.; ORTÍZ, H.; FAZZINI, P.

Toronto, Canadá

Conferencia; Second International Conference on Engineering Failure Analysis. ICEFA-II.; 2006

Metallurgical susceptibility to intergranular cracking was assessed, after repeated failures of thick wall curved steel tubes in a polymerization reactor. Cracked tubes are made of HP-4 steel, bent and heat treated, and then subjected to autofrettage. Service pressure is 250 MPa, temperature around 325C.  All failures are characterized by multiple intergranular and branched cracks, mostly circumferential and Most cracks initiated in the outer surface, in contact with process steam, and were initially related to stress corrosion cracking, SCC. Other cracks initiated in pre existing defects in the inner surface of the tube, in contact with polymer. Oddly enough, yet another tube failed only after one month in service, with cracks initiated in the mid thickness of the tube wall. It was apparent that the material of the reactor tubes suffered embrittlement mechanisms, therefore, a global study was carried out, that included the experimental assessment of deformation and temperature induced embrittlement mechanisms. Longitudinal residual stresses were measured, mechanical testing included tensile, Charpy and SCC tests, followed by fractographic, and metallographic analyses.  Susceptibility to intergranular cracking was experimentally assessed by recreating conditions of embrittlement by thermal treatments and tensile testing.  Samples with 0, 3 and 5% plastic deformations were subjected to 24 hr. thermal treatments between 300 and 400 ºC. It was found that under the combination of plastic deformation during fabrication and later service temperatures the material loses strength just after yielding. In this condition the material is unable to sustain stresses above yielding under conditions of restricted plastic deformation, as occurs in the plane deformation stress states. These occur in presence of defects and in the mid thickness. This does not occur in the material without previous plastic deformation (not subjected to autofrettage)